Spectral Responsivity Calibration of UV Photometric Broadband Imagers for the Carruthers Geocorona Observatory Mission

The Carruthers Geocorona Observatory is NASA's first mission dedicated to investigating the fundamental nature of Earth’s exosphere. Its primary payload, the GeoCoronal Imager, consists of two co aligned broadband photometric imagers that support simultaneous, common-volume sensing of ultraviolet emission by exospheric hydrogen atoms. As with any photometric system, accurate parameter retrieval requires precise knowledge of the instrument’s wavelength dependent responsivity. To that end, we first assemble a refined UV stellar spectral library tied to CALSPEC standards. We then derive each imager’s effective responsivity across the passband by performing photometric measurements of a subset of these stars and inverting the observed fluxes. Additionally, we introduce an objective, algorithm driven ranking criterion to identify the best subset that most effectively constrain the passband retrieval. The entire workflow—including the spectral library, passband inversion, and selection criterion—is validated using synthetically generated stellar measurements.